There is high industrial demand for transparent, high-gloss plastic films, e.g. biaxially oriented polypropylene films or biaxially oriented polyester films. Alongside this, there is increasingly a requirement for these transparent films and moldings where protection is afforded from ultraviolet radiation and where at least one surface layer does not have high gloss but has a characteristic matt appearance, giving the application or the moldings an appearance which is particularly attractive and therefore effective for promotional purposes, and giving them protection from UV radiation.
EP 346 647 describes a biaxially oriented polyester film which comprises at least one outer layer which comprises a concentration of from 0.5 to 50% of a filler, the diameter of this filler having a particular ratio to the thickness of the outer layer. The outer layer also has a particular thickness and a particular degree of crystallization, determined with the aid of Raman spectroscopy.
U.S. Pat. No. 4,399,179 describes a coextruded biaxially oriented polyester film which is composed of a transparent base layer and of at least one matt layer which essentially consists of a particular polyethylene terephthalate copolymer, and also comprises a concentration of from 3 to 40% of inert particles with diameter of from 0.3 to 20 μm. The specific copolymer is a processing aid which reduces the viscosity of the melt comprising the inert particles, permitting satisfactory extrusion of this layer. Addition of the inert particles to the appropriate layer gives the film its mattness. The particles impair transparency.
EP 0 144 978 describes a self-supporting oriented film made from thermoplastic and bearing, on at least one of its two surfaces, a continuous polyester coating which is applied in the form of an aqueous dispersion to the film prior to the final stretching step. The polyester coating is composed of a condensation product of various monomers or of their derivatives capable of forming polyesters, for example isophthalic acid, aliphatic dicarboxylic acid, sulfomonomers, and aliphatic or cycloaliphatic glycol.
EP-A-0 620 245 describes films with improved heat resistance. These films comprise antioxidants suitable for scavenging free radicals formed within the film and degrading any peroxide formed. That specification does not, however, provide any proposal as to how the UV resistance of these films may be improved.
The prior art described gives no indication that the films are thermoformable, or as to how at least one surface of the film may be provided with low gloss while retaining high transparency, or that the film absorbs UV light, or that the film has high UV resistance.
It was an object of the present invention, therefore, to provide a coextruded biaxially oriented and transparent polyester film which has at least one matt outer layer and is simple and cost-effective to produce, and has the good physical properties of known films, and moreover in particular absorbs UV light and has high UV resistance, and also has good thermoformability. The film should also cause no problems of disposal.